1. Field of the Invention
The present invention relates to a compound semiconductor device employing an SiC substrate and a method for producing the same. In particular, the present invention relates to a GaAlInN series compound semiconductor light emitting device and a method for producing the same.
2. Description of the Related Art
A GaAlInN series compound semiconductor is a wide gap semiconductor and has a band structure of a direct transition type. It is expected to be used in developing a light emitting device which emits light having a wavelength in the range of blue to ultra violet (UV). FIG. 12 shows a double-hetero type LED (light emitting diode) which has been used in practice as a gallium nitride series light emitting device. The LED is produced as follows.
First, a GaN or AlN buffer layer 102 is deposited to be about 200 .ANG.-thick on a sapphire C-surface substrate 101 at a substrate temperature of about 600.degree. C. by using an metal organic chemical vapor deposition method (referred to as the "MOCVD method" hereinafter).
Then, an n-type GaN layer 103 for grating matching and an n-type GaAlN cladding layer 104 are grown to be about 4 .mu.m-thick and about 0.5 .mu.m-thick, respectively, on the buffer layer 102 at a substrate temperature of about 1050.degree. C.
Next, the substrate is cooled down to about 800.degree. C., and a Zn-doped GaInN light emitting layer 105 is grown to be about 0.05 .mu.m-thick on the cladding layer 104. Then, the substrate is again heated to about 1050.degree. C., and an Mg-doped GaAlN cladding layer 106 and an Mg-doped GaN contact layer 107 are grown to be about 0.5 .mu.m-thick and about 0.3 .mu.m-thick, respectively, on the light emitting layer 105. After growing these layers, the semiconductor device is taken out of an MOCVD apparatus, and is subjected to a heat treatment in a nitrogen atmosphere at about 600.degree. C. for about 30 minutes so as to lower the resistances of the Mg-doped GaAlN cladding layer 106 and the Mg-doped GaN contact layer 107 and to form these layers as p-type. Then, for forming an n-type electrode, portions of the semiconductor device are etched off by using an RIE (reactive ion etching) method until a portion of the n-type GaN layer 103 is exposed. Finally, an n-side electrode 108 is formed on the exposed portion of the n-type GaN layer 103, and a p-side electrode 109 is formed on the p-type GaN contact layer 107, thus producing the LED.
However, a gallium nitride series semiconductor device employing a sapphire substrate has the following problems.
There is a considerable difference between the grating constant of the sapphire substrate and that of the GaN layer. Accordingly, there exists a lattice mismatching of about 16% or more, and the resulting misalignment at the interface between the sapphire substrate and the GaN layer leads to a grating defect in the GaN layer. For reducing the grating defect, it has been proposed, as in the conventional technique above, to insert a GaN or AlN buffer layer grown at a relatively low temperature of about 600.degree. C. between the sapphire substrate and the GaN layer, or to clean the surface of the substrate by using a chemical etching method. However, there still exists a large amount (about 10.sup.10 to 10.sup.11 cm.sup.-2) of grating defect at the surface of the buffer layer, thus deteriorating the performance of the LED at high temperatures and hence the reliability of the device.
In the gallium nitride series light emitting device, it has been proposed to employ, an SiC substrate in place of a sapphire substrate since the lattice constant of the SiC substrate is relatively close to that of a GaN layer as compared to a saphire substrate. Despite the relatively close grating constants, however, laser oscillation has not been realized in a gallium nitride series semiconductor laser device employing the SiC substrate. Therefore, it has also been proposed to form an AlN buffer layer on the SiC substrate for improving the lattice matching therebetween. FIG. 13 shows the structure of the GaAlInN series semiconductor laser device employing the SiC substrate. The production process of the GaAlInN series semiconductor laser device will be now described.
First, an AlN buffer layer 203 is grown to be about 0.2 .mu.m-thick on an SiC substrate 201 at a substrate temperature of about 1100.degree. C. by using an MOCVD method. Next, an n-type GaN layer 204 and an n-type GaAlN lower cladding layer 205 are formed to be about 4 .mu.m-thick and about 0.5 .mu.m-thick, respectively, on the AlN buffer layer 203 at a substrate temperature of about 1050.degree. C. Then, the substrate is cooled down to about 800.degree. C., and a GaInN active layer 206 is grown to be about 100 .ANG.-thick on the lower cladding layer 205. Then, the substrate is again heated to about 1050.degree. C., and an Mg-doped GaAlN cladding layer 207 and an Mg-doped GaN contact layer 208 are grown to be about 0.5 .mu.m-thick and about 0.3 .mu.m-thick, respectively, on the active layer 206.
After growing these layers, the semiconductor device is taken out of the MOCVD apparatus, and is subjected to a heat treatment in a nitrogen atmosphere at about 600.degree. C. for about 30 minutes so as to lower the resistances of the Mg-doped GaAlN cladding layer 207 and the Mg-doped GaN contact layer 208 and to form these layers as p-type layers. Then, an Al.sub.2 O.sub.3 film 210 including apertures 209 in strips is formed by using an electron beam deposition method and a photolithography process. A p-side electrode 211 is formed so as to entirely cover the Al.sub.2 O.sub.3 film 210 and the Mg-doped GaN contact layer 208. An n-side electrode 212 is formed over the entire bottom surface of the SiC substrate 201. The GaAlInN series semiconductor laser device is thus produced.
Such a semiconductor laser device has the following problems.
The AlN buffer layer is deposited on the SiC substrate for improving the lattice matching. However, the defect at the surface of the AlN buffer layer is still about 10.sup.8 to 10.sup.10 cm.sup.-2. This is merely a 10.sup.2 cm.sup.-2 reduction from the defect between the AlN buffer layer and the sapphire substrate. Thus, light and carriers are not satisfactorily confined within the active layer. Moreover, crystalline quality sufficient to undergo a carrier injection of about 10.sup.19 cm.sup.-3 required for laser oscillation has not been realized. Thus, laser oscillation by current injection has not been realized.
As described above, the gallium nitride series semiconductor material is strongly expected to offer a suitable material for a light emitting device which emits light having a wavelength in the range of blue to UV. However, a production method which can sufficiently reduce the grating defect in a layer of the gallium nitride series compound has not been realized and, therefore, laser oscillation by current infusion has not been realized.